1. Field of the Invention
The present invention relates to a circuit module that is included in a communication apparatus for use in a high-frequency band and includes a multilayer substrate including built-in capacitors.
2. Description of the Related Art
As communication apparatuses, such as mobile telephones, decrease in size and weight and increase in functionality, circuit elements embedded in these communication apparatuses have been reduced in size and increased in functionality. There is also an increasing demand for a compact and multifunctional circuit module including such a circuit element.
Each circuit module used in communication apparatuses, such as mobile telephones, is generally produced with a multilayer substrate including built-in capacitors. The multilayer substrate including built-in capacitors is obtained by laminating a plurality of functional layers. An active element and a passive element, which are required to form a circuit module, are mounted on each of the functional layers.
An example of such a circuit module is disclosed in Japanese Unexamined Patent Application Publication No. 2005-39263.
The configuration of the circuit module disclosed in Japanese Unexamined Patent Application Publication No. 2005-39263 will be described with reference to FIGS. 4, 5, and 6.
FIG. 4 is a cross-sectional view of a circuit module 100. The circuit module 100 includes a multilayer substrate 101 including built-in capacitors. The multilayer substrate 101 including built-in capacitors is obtained by laminating a plurality of functional layers.
FIGS. 5 and 6 are diagrams illustrating functional layers 106 and 107, respectively. FIG. 5 is a plan view of the functional layer 106 as viewed from the surface of the functional layer 106. On the surface of the functional layer 106, capacitor electrodes C13 to C16 and inductor electrodes L1 and L2 are formed. One end of the inductor electrode L1 is connected to the capacitor electrode C13, and the other end of the inductor electrode L1 is arranged close to the capacitor electrode C15. One end of the inductor electrode L2 is connected to the capacitor electrode C14, and the other end of the inductor electrode L2 is arranged close to the capacitor electrode C16.
FIG. 6 is a plan view of the functional layer 107 as viewed from the surface of the functional layer 107. On the surface of the functional layer 107, capacitor electrodes C131, C141, C151, and C161 are formed. The capacitor electrodes C131, C141, C151, and C161 face the capacitor electrodes C13, C14, C15 and C16, respectively, formed on the surface of the functional layer 106 via the functional layer 106, so that capacitors C3 to C6 are formed in the functional layer 106 that is a capacitor layer.
A design method of providing a good characteristic for a circuit module by adjusting the value of a capacitor in a circuit to an optimum value is often used.
For example, in the case of the multilayer substrate 101 including built-in capacitors in the circuit module 100, the area of the capacitor electrode C15 may be changed so as to adjust the capacitance value of the capacitor C5. In this case, however, since the inductor electrode L1 and the capacitor electrode C15 are close to each other on the same dielectric layer, the state of coupling between the inductor electrode L1 and the capacitor electrode C15 is also changed. In a case in which the thickness of the functional layer 106 is changed so as to change the capacitance value of the capacitor C5, the capacitance values of the other capacitors in the functional layer 106 that is a capacitor layer, that is, the capacitors C4, C5, and C6, are also changed.
Such a change in a capacitance value will be described with reference to FIGS. 7A and 7B. Referring to FIG. 7A, capacitor electrodes 1 and 2 face each other, so that a capacitor C1 is formed. An inductor electrode 3 is formed over the capacitor electrode 1, and a coupling capacitor C2 is formed between the inductor electrode 3 and the capacitor electrode 1. FIG. 7B is a diagram illustrating the change in the area of the capacitor electrode 1 which is performed so as to change the value of the capacitor C1. As illustrated in the drawing, the reduction in the area of the capacitor electrode 1 changes the value of the coupling capacitor C2 between the capacitor electrode 1 and the inductor electrode 3. Although not illustrated, in a case where the position of the capacitor electrode 2 in the lamination direction is changed so as to change the value of the capacitor C1, the value of the coupling capacitor C2 is also changed.
Thus, in a multilayer substrate including built-in capacitors in the related art, when the value of a single capacitor in the substrate is optimized, the state of coupling between a corresponding capacitor electrode and another circuit element in the substrate is also changed. Accordingly, it is necessary to change the overall design of the multilayer substrate.